Because Malaysia Airlines is making wearing/having a mask mandatory for all passengers (except infants), effective tomorrow (23 April). So if you don't have a mask, you don't get to fly. You won't be allowed to check in, let alone board your flight.

"So, we can't be enforcing this law against anyone who is not wearning a face mask and detain or arrest them and such. This is the same scenario when a shopping centre guard stopped an individual from entering the premises to purchase food because he was not wearing a mask This goes beyond the law and the Act. This goes beyond what is need ed and what is necessary. It is not an offence if you are not wearing the face mask."